What are Saturn's rings made of?

Scientists have pondered Saturn's rings since Galileo peered at the planet through an early telescope in 1610. From the angle at which he observed it, Galileo surmised that Saturn wasn't a single star, but was actually three: a large middle star with two earlike appendages sticking out of it, which he thought might be large moons. Galileo observed Saturn for more than a year. Then he took a break and didn't look again until 1612, when he saw something unusual. Instead of the three-star formation that he had seen on his last viewing, Galileo saw just a single star. He correctly predicted that the other "stars" would return, but he couldn't figure out why they had disappeared.

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In 1655, Dutch scientist Christiaan Huygens answered the question that had so puzzled Galileo when he looked through a more sophisticated telescope. He decided that the extra stars were actually rings, which were thin enough to seemingly disappear when viewed on their edge. Today, scientists have a name for what Galileo and Huygens witnessed -- the ring plane crossing. As Saturn travels around the sun, its rings appear edge-on to the Earth about once every 14 years. So when we look at the planet through a telescope during that time, the rings aren't visible.

Huygens made one mistake in his evaluation of Saturn, however. He believed that the rings were solid. Five years later, French astronomer Jean Chapelain more accurately surmised that the rings were actually small particles orbiting around Saturn. Scottish physicist James Clerk Maxwell confirmed this theory in 1857 when he figured out that the rings had to be made of small particles; otherwise, they would be pulled inward by Saturn's gravity until they crashed into the planet.

In the 20th and 21st century, astronomers have had the benefit of technology to help them discover the secrets of Saturn's rings. In the late 1970s and early '80s, the Pioneer and Voyager spacecrafts sent back close-up views of the rings and the particles that compose them. In recent years, the Cassini mission (a collaborative effort between NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI)), has been able to circle even closer to Saturn's rings and gather a great deal of new information about their structure.

As they have learned more and more about the composition of Saturn's rings, scientists have also been questioning the rings' origins. They believe that the rings were created when comets or asteroids collided with one or more of the planet's moons, shattering them into many pieces. The fragments from the collision spread out around Saturn and formed into the current ring pattern.

What isn't for sure is the age of the rings. At first they were thought to be as old as the solar system. Then scientists surmised that the ice in the rings should be far dirtier than it was if it had been gathering space dust for 4 billion years. They consequently moved the estimated age of the rings forward to tens of millions of years ago. But when the Cassini spacecraft sent back the clearest images yet of Saturn's rings, scientists said the original estimate may have been correct after all. They believe it's likely that the ring particles were recycled over 4 billion years, and that they will continue to exist long into the future.

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